1. Field of the Invention
The present invention relates to measurement of electromagnetic interference (EMI), and more particularly to high-precision automated techniques for testing and reporting EMI emissions from radiating objects.
2. Description of the Related Art
Electromagnetic interference (EMI) from radiated emissions and from conducted emissions is problematic in a wide variety of contexts. For example, power transmission lines may convey transient surges or other voltage irregularities to electrical equipment intended to be powered therefrom, such that equipment in line therewith may cause an interference to an intervening electrical bus and to the power lines themselves. This interference may compromise computer performance, television picture quality, or the functioning of other electrical equipment powered from such lines. Electromagnetic energy radiated from the equipment itself often causes severe, undesirable effects which range from poor signal quality or poor reception in radios and televisions, to complete ineffectiveness or inoperability of electronic devices.
Electronic devices, most notably electronic communication devices, typically emit some amount of undesirable EMI. These emissions may be a concern for a number of reasons. EMI emissions can potentially interfere with devices communicating in the same frequency band. As the frequency spectrum available for communications becomes increasing crowded to meet demands for wireless communications, the importance of minimizing EMI emissions from individual devices becomes more critical. Presently, for example, U.S. Federal Communications Commission (FCC) requirements mandate that every cellular communications telephone be tested to certify EMI performance. Similarly, certain telecommunications equipment for government or military applications require strict conformance to military emissions standards.
More rigorously, EMI can broadly be divided into two main categories: natural and man-made. The former is due to atmospheric effects and can be divided into low frequency electric and magnetic fields and high frequency electromagnetic fields. The source of man-made interference is due to radio transmitters, including harmonics and spurious frequencies resulting from mixing intermediate frequencies, electroheating elements (microwaves) and digital computational devices increasingly being used in radio transceivers. Classes of interference which are specifically regulated in radiowave transmission devices include harmonics and spurious interference. For example, military standards (MIL-STD-261 E) require the following suppression:
Transmit Harmonics:xe2x88x9250 dBc for 2nd and 3rd, xe2x88x9280 dBc for all others
Transmit Spurious:xe2x88x9280 dBc,
where dBc is the interference power referenced to the carrier, i.e., the 2nd and 3rd harmonics must be 50 dB below the carrier at the antenna output. The spurious outputs are produced by intermodulation products in the IF path (for both receive and transmit).
Another concern is that electromagnetic energy radiated or conducted from electronic devices, including EMI emissions, generally contains information, which may be extractable by unintended parties. In particular, certain electronic devices not designed to emit electromagnetic signals (e.g., personal computers and conventional telephones) or designed to emit only short-range signals may nevertheless emit significant EMI signals detectable at some distance from the device. For security reasons, such devices may require certain shielding in order to prevent such devices from emitting radiation in a manner that allows an individual monitoring the emitted radiation to discern intelligible information regarding the content of the communication. Such concerns are particularly relevant to government or military systems and devices. Moreover, with the rising specter of commercial espionage and its harmful impact on commercial businesses, industries within the private sector are also placing increased emphasis on EMI testing.
To ensure compliance with EMI requirements, EMI testing is performed in a wide variety of commercial and military contexts. Currently, EMI certification is highly manpower intensive. Analysts often conduct tests literally by hand, with little or no process automation. This situation is quite similar to that faced by surveillance personnel over the last forty years where surveillance analysts in signal intelligence activities (SIGINT) spend countless hours examining monitors in an attempt to identify by sight and/or sound, signals of interest. On a reduced scale, fields of radio astronomy, as well as geophysics and biomedicine, also encounter similar manpower-intensive signal testing.
Presently, EMI testing of electronic devices is generally performed using analog equipment. Basically, a conducted or radiated signals is detected by means of an antenna attached to an analog radio frequency (RF) receiver, and the peak voltage of a time domain signal is registered (by hand) and compared to a mask or threshold provided by various standards (e.g., military or commercial standards). If the peak voltage exceeds the threshold, the electronic device(s) under test is said to fail the specific test.
These tests are generally conducted at different bandwidths in a total span of 1 GHz. More specifically, two different types of tests are conducted; a broadband scan test and a narrowband scan test. In the broadband scan, the peak voltage is measured in a 100 kHz bandwidth. In the narrowband scan, different bandwidths are selected and range from 0.5 kHz, 1.0 kHz, 5.0 kHz, 10 kHz, 25 kHz, 50 kHz, 75 kHz and 100 kHz. This process is repeated through each 100 kHz bandwidth in the total 1 GHz bandwidth.
For example, consider a conventional EMI test involving analysis of a 1 GHz bandwidth in segments of 1 kHz. The process requires assessing the signal bandwidth in 10 kHz segments looking for spurious 1 kHz tones which are audio identified by an analyst. The next step is to measure the amplitude of the audio tone, which is done by reconnecting the system (i.e., reconnecting cables). An experienced analyst can analyze between three to seven segments per second, not inclusive of the time required to measure the amplitude of detected signals; consequently, the time required to analyze the entire 1 GHz bandwidth can be on the order of eight hours and is highly susceptible to human error. Long testing times increase the duration of product development cycles and increase unit production time and cost, potentially impacting timely delivery of products.
Moreover, such a process requires expensive analog equipment, given the number of filters which have to be applied at the front end to select each one of the different bandwidths. Furthermore, each bandwidth scan has to be run separately and there is no permanent record of the final reading (i.e., the comparison with the mask or threshold). Thus, analog testing does not provide a xe2x80x9chistoryxe2x80x9d of testing that can be later reviewed or referred to, and problems that arise during testing typically have to be solved over and over again, since only the memory of analysts can be relied upon to recall and address testing problems.
Accordingly, it would be highly desirable to implement an automated approach to EMI testing using digital signal processing techniques for detecting, analyzing, identifying, and quantifying the amplitude level of EMI signals produced from a radiating body.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to automate EMI testing using digital signal processing techniques to thereby eliminate the need for human observation of signals and resulting errors.
A further object of the present invention is to reduce the time required to perform EMI testing of equipment.
Another object of the present invention is to enhance the accuracy of EMI testing.
Yet a further object of the present invention is to reduce the number of analog components required in EMI testing equipment to thereby reduce equipment size, weight and cost.
A still further object of the present invention is to permit recordation and storage of EMI measurements and signal processing parameters.
Yet another object of the present invention is to enable EMI testing to be performed off-line with recorded signals, such that EMI testing results are repeatable.
Still another object of the present invention is to have the capability to apply different signal filtering to recorded EMI measurements to refine assessments of EMI conditions.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, an EMI testing scheme involves analyzing an analog input signal in frequency band segments. The received analog input signal is frequency down-converted to baseband, passband filtered and digitally sampled to form digital signals representative of the analog input signal. For example, the bandwidth of the signal under test can be 1 GHz. The digital signal is supplied to a digital signal processor which converts the signal to a complex analytic signal via a Hilbert transform filter and then transforms the analytic signal into the frequency domain via a Discrete Fourier Transform (DFT). Pre-stored frequency window filters are applied to the complex frequency domain signal. The set of discrete filter sample points which form the window filters is designed such that particular narrow-bandwidth sub-bands within the frequency segment are sequentially selected for EMI analysis. By applying different frequency window filters to the frequency domain signal, different frequency sub-bands are selected for analysis. The filtered frequency domain signal is transformed back to the time domain, by means of an inverse DFT, and the peak voltage of the time domain signal is compared with a threshold to determine whether the EMI levels within the selected sub-band are acceptable.
The digital signal processing techniques of the present invention allow recordation of digitally sampled signals and processing parameters, thereby permitting precise repeatability of EMI tests and extensive off-line analysis. This capability aids in test and development of new algorithms and techniques. Another important improvement of the present invention over the prior art EMI testing is a dramatic reduction in the time required to complete EMI testing, thereby shortening product development and production time. By automating certain test functions which have been performed manually by an experienced or certified operator or EMI technician, improvements of orders of magnitude are possible. The digital signal processing techniques of the present invention can be used to recognize a particular signal or its given variations, so that the number of bandwidths that can be scanned per second is limited only by the data collection rate and algorithm processing speed. Digital signal processing also reduces the risk of human error in detecting EMI signals of concern.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.